This invention relates generally to gas coupled gas turbine engines and, more particularly, to a mounting system for the power turbine shaft and first stage reduction gear set in such engines.
2. Description of the Prior Art
In typical gas coupled gas turbine engines, a power turbine is supported on an engine block of the engine for rotation about a primary axis of the block independently of the gasifier shaft of the engine. The gasifier produces a continuous stream of hot gas motive fluid which is directed by a power turbine nozzle against an array of blades on the power turbine whereby a power turbine shaft is rotated at high speeds of on the order of 100,000 RPM in some small engines. Torque is transferred from the power turbine shaft to an output shaft of the engine by one or more reduction gear sets including a first stage pinion gear rotatable as a unit with the power turbine shaft and a first stage reduction gear rotatably supported on the engine block. Typically, the first stage pinion gear is rigidly connected directly to the power turbine shaft which is supported on the gine block through either a two or three bearing mounting system. In two bearing mounting systems, the power turbine shaft is supported on the engine block by bearings at each end of the shaft, the first stage pinion gear being rigidly mounted on the shaft near one of the bearings. While such systems adequately support the power turbine shaft, the softness of at least one of the bearing mounts necessary to control shaft vibrations renders it difficult to achieve and maintain the highly accurate mesh between the first stage pinion gear and the first stage reduction gear which is required for durability where the pinion gear rotates at on the order of 100,000 RPM. In three bearing mounting systems, a third bearing between the engine block and the power turbine shaft cooperates with the one of the power turbine shaft bearings nearest the first stage pinion in more rigidly and accurately straddle mounting the pinion gear on the engine block. The third bearing, however, renders it more difficult to mount the power turbine shaft on the engine block and also may render it more difficult to achieve adequate vibration damping. A power turbine shaft and first stage reduction gear assembly according to this invention incorporates, in one system, advantages of both the two and the three bearing mounting systems.